Refining highly aromatic lube oil stocks

Abstract

A process for solvent extraction of higher aromatic hydrocarbon oils with N-methyl-2-pyrrolidone wherein N-methyl-2-pyrrolidone is separated from a solvent-extract mix in a series of distillation steps with recirculation of a portion of a partially stripped extract mix from an intermediate distillation step to the extraction step to improve the solvent-oil ratio in the extraction step and conserve energy as compared with conventional processes in which the solvent power of N-methyl-2-pyrrolidone is moderated by incorporating water in the solvent.

We claim: 
     
       1. In a process for solvent refining a petroleum based oil stock containing aromatic and non-aromatic components wherein said oil stock is contacted with N-methyl-2-pyrrolidone in a solvent extraction zone to produce an aromatics-rich primary extract phase containing solvent and an aromatics poor raffinate phase containing solvent, said phases are separated from one another and separately processed for the recovery of solvent therefrom for recycle to the process, the improvement which comprises returning a portion of said aromatics-rich extract to said solvent extraction zone in admixture with said recycle solvent. 
     
     
       2. A process according to claim 1 wherein said aromatics-rich extract returned to said extraction zone in admixture with recycle solvent comprises N-methyl-2-pyrrolidone in an amount less than that contained in said primary extract phase. 
     
     
       3. A process according to claim 1 wherein the volume of extract returned to said solvent extraction zone is sufficient to provide 0.5 to 3 weight percent extract reckoned on a solvent-free basis in admixture with N-methyl-2-pyrrolidone supplied to said solvent extraction zone. 
     
     
       4. A process according to claim 3 wherein the volume of extract returned to said solvent extraction zone is sufficient to provide 1 to 2 weight percent extract in admixture with said N-methyl-2-pyrrolidone supplied to said solvent extraction zone. 
     
     
       5. A process according to claim 1 in which the contacting temperature in said solvent extraction zone is within the range of 50° to 80° C. 
     
     
       6. A process according to claim 1 in which solvent is separated from said extract phase in a series of distillation steps and said aromatics rich extract returned to said extraction zone is a fraction withdrawn from an intermediate step in said series and comprises N-methyl-2-pyrrolidone. 
     
     
       7. In a process for solvent extraction of a hydrocarbon lubricating oil feedstock effecting separation of said hydrocarbon feedstock into a raffinate product of reduced aromaticity and an extract product of increased aromaticity which comprises (a) contacting said hydrocarbon mixture with N-methyl-2-pyrrolidone as solvent for aromatic hydrocarbons in an extraction zone forming a raffinate phase comprising said raffinate product with a minor portion of said solvent dissolved therein and a primary extract phase comprising said solvent with said extract product dissolved therein,   (b) passing said primary extract phase to a first distillation zone at a pressure in the range of 1 to 5 psig effecting separation of a portion of said solvent as distillate from distillation bottoms comprising said extract, and   (c) heating said bottoms from said first distillation zone and passing said heated bottoms to a second distillation zone maintained at a pressure in the range of 40 to 50 psig wherein an additional portion of said solvent is separated as a distillate from distillation bottoms comprising said extract and at least one third of said portion of said solvent, the improvement which comprises   (d) admixing a portion of said second distillation bottoms with N-methyl-2-pyrrolidone solvent supplied to said extraction zone in an amount sufficient to form a solvent feed stream containing 0.5 to 3 weight percent of said extract calculated on a solvent-free basis.